How come I need to manually correct .mtl file?
The answer is: It depends.

The .mtl file is following the specification and is therefore correct. However some viewers do not implement the specification in its entirety or may not even implement it correctly. Therefore it may be necessary to adjust the .mtl file to make it work properly. However this is not because the .mtl is wrong, but because the viewer doesn't support the Ns, Ks and illum statements.


  Ns exponent
  
  Specifies the specular exponent for the current material.  This defines 
  the focus of the specular highlight.
  
  "exponent" is the value for the specular exponent.  A high exponent results in a tight, concentrated highlight.  Ns values normally range from 0 to 1000.





  Ks r g b 
  
  The Ks statement specifies the specular reflectivity using RGB values.
  
  "r g b" are the values for the red, green, and blue components of the atmosphere.  The g and b arguments are optional.  If only r is  specified, then g, and b are assumed to be equal to r.  The r g b values  are normally in the range of 0.0 to 1.0.  Values outside this range  increase or decrease the relectivity accordingly.





  illum illum_#
  
  The "illum" statement specifies the illumination model to use in the material. Illumination models are mathematical equations that represent various material lighting and shading effects.
  
  "illum_#" can be a number from 0 to 10.  The illumination models are summarized below; for complete descriptions see "Illumination models" on page 5-30.
  
  Illumination    Properties that are turned on in the Property Editor
  
  0     Color on and Ambient off
  
  1     Color on and Ambient on
  
  2     Highlight on
  
  3     Reflection on and Ray trace on
  
  4     Transparency: Glass on
        Reflection: Ray trace on
  
  5     Reflection: Fresnel on and Ray trace on
  
  6     Transparency: Refraction on
        Reflection: Fresnel off and Ray trace on
  
  7     Transparency: Refraction on
        Reflection: Fresnel on and Ray trace on
  
  8     Reflection on and Ray trace off
  
  9     Transparency: Glass on
        Reflection: Ray trace off
  
  10        Casts shadows onto invisible surfaces